STUDY OF THE RELATIONSHIP BETWEEN STRUCTURE AND O-17 ELECTRIC FIELD GRADIENT PARAMETERS IN SOME ALUMINOSILICATES

The relation between O-17 quadrupolar parameters and the local structure in silicates is of considerable interest if these parameters are to be used as a local probe. Although there are a number of investigations and some theoretical work on Si—O—Si bonds, there seems to be very little experimental data on Si—O—Al bonds. We have therefore investigated a number of sodalites with a general formula of M8[AlSiO4]Cl2 where M is an alkali cation. They have only one oxygen site and a Si:Al ratio of one. By varying the alkali we were able to change the Si—O—Al bond angle about five degrees steps from 125 to 152 degrees. The value of the electric field gradient (EFG), CQ of O-17, followed a roughly similar angular dependence to that found for Si—O—Si bonds with CQ180= 4.2 MHz. The asymmetry parameter of O-17 also behaved in a similar fashion to that found for Si—O—Si bonds although it increased more rapidly at small angles.